This application relates generally to detecting frequency modulated (FM) signals and more particularly to detecting signals from FM altimeters and radars.
FM signals are well suited for measuring the distance from a transmitter to an object from which the FM signal reflects. Signals in which the frequency increases or decreases linearly ("chirped signals") are particularly well suited for these functions. For this reason, FM signals are sometimes used in altimeters on aircraft. Also, FM signals are used in radars which are often on aircraft or other moving bodies.
It is sometimes desirable to determine when an aircraft is in the vicinity. For example, in a battlefield scenario, it can be very important to detect all aircraft in the area. The FM signals emitted by altimeters or radars on aircraft can indicate the presence of the aircraft.
However, a problem results when FM altimeter or FM radar signals are used to detect aircraft. These signals are generally very directive and are not always directed at the device trying to detect the aircraft. For example, an altimeter signal is transmitted directly at the ground below the aircraft. A device trying to detect the aircraft would receive only the side lobes of the FM signal. Thus, the received FM signal would be at a relatively low level.
Detecting relatively low level signals, especially in noisy environments, can be difficult and requires a high sensitivity receiver. Existing systems use what is called a "walk-through receiver" to detect FM signals. The walk-through receiver contains a relatively narrow bandpass filter. As the FM signal changes in frequency, its frequency will, for some period of time, fall in the passband of the filter. The output of the filter will be a pulse having a non-zero value when the FM signal has a frequency in the passband of the filter. Ordinary pulse detection techniques are applied to the output of the filter to detect the pulses which signify the presence of an FM signal.
The walk-through receiver has a drawback in that only a portion of the energy in the FM signal appears in the output pulse. In effect, the signal level is attenuated. This attenuation reduces the sensitivity of the receiver and makes detection of an FM signal less likely.